The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, the approaches described in this section may not be prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
In semiconductor integrated circuit manufacturing, it is conventional to test integrated circuits (“IC's”) during manufacturing and prior to shipment to ensure proper operation. Wafer testing is a well-known testing technique commonly used in production testing of wafer-mounted semiconductor IC's, wherein a temporary electrical connection is established between automatic test equipment (ATE) and each IC formed on the wafer to demonstrate proper performance of the IC's. Components that may be used in wafer testing include an ATE test board, which is a multilayer printed circuit board that is connected to the ATE, and that transfers the test signals between the ATE and a probe card assembly. The probe test card assembly (or probe card) includes a printed circuit board that generally contains several hundred probe needles (or “probes”) positioned to establish electrical contact with a series of connection terminals (or “die contacts”) on the IC wafer. Conventional probe card assemblies include a printed circuit board, a substrate or probe head having a plurality of flexible test probes attached thereto, and an interposer that electrically connects the test probes to the printed circuit board. The interposer conventionally includes telescopic “pogo pins” or solder bumps that provide electrical connections between conductive pads on the printed circuit board and the interposer and between the interposer and conductive pads on the substrate. The test probes are conventionally mounted to electrically conductive, typically metallic, bonding pads on the substrate using solder attach, wire bonding or wedge bonding techniques.
In some applications, multiple test probes need to be applied to a single pad or bump to perform a test. For example, in RF or switching applications, two test probes that are electrically insulated from each other are needed to establish an RF current path. One test probe carries the RF forward signal and the other test probe carries the RF return signal, which is commonly referred to as a ground path if the ground is used as the return path. As another example, Kelvin connections use four probes. Two probes are used to apply a test signal and the other two probes are used to measure a voltage.
Using multiple conventional test probes to contact a single pad or bump has proven to be difficult for several reasons. Sometimes the test pad or bump presents an uneven surface that makes it difficult to maintain contact. For example, conventional test probes have a tendency to misalign or slip off bumps, causing an open circuit. Even if contact can be maintained, conventional test probes are relatively large compared to test pads and bumps. This requires that the conventional test probes be spaced apart and angled so that the tips can make contact with the test point. The distance between the probes, probe cross sectional area and probe length all have a direct effect on the inductance of the forward and reverse paths. A larger probe pitch, larger cross sectional area and longer probe length generally cause an increase in the path inductance, which is undesirable in RF or switching applications. Also, two test probes may exert different contact forces on a test point, which causes uneven tip wear and also uneven scrubbing on the test point. Conventional test probes are therefore generally not well suited for these types of applications. Based on the foregoing, there is a need for a test probe assembly that does not suffer from limitations of conventional test probes.